1. Field of the Invention
This invention relates to the preparation of polyamines and is more particularly concerned with an improved process for the condensation of aniline and formaldehyde to yield polyamine mixtures.
2. Description of the Prior Art
The preparation of the mixtures of methylene-bridged polyphenyl polyamines (hereafter polymethylene polyphenyl polyamines) containing di(aminophenyl)methanes by condensation of aniline and formaldehyde under aqueous conditions in the presence of strong acids, particularly hydrochloric acids, has been widely described. Illustrative of such processes are those shown in U.S. Pat. Nos. 2,638,730; 2,950,263; 3,260,751; 3,277,173; 3,297,759 and 3,476,806. Such processes, in one form or another, are widely used commercially to prepare the polyamines in question, which polyamines are employed as intermediates in the preparation of the corresponding isocyanates, i.e. diisocyanatodiphenylmethane and mixtures of polymethylene polyphenyl polyisocyanates. The isocyanates are employed in the preparation of a variety of polyurethanes, polyisocyanurates and other polymers (both cellular and non-cellular) which can be derived from polyisocyanates.
For many purposes it is desirable that the diamine content of the above polyamines, and the diisocyanate content of the polyisocyanates derived therefrom, contain a high proportion of 4,4'-isomer and this has been achieved largely by employing proportions of strong acid in the condensation of the aniline and formaldehyde with the aniline present in an amount of at least 2 mole per mole of formaldehyde. If the amount of acid is reduced significantly below the above minimum, the proportion of 4,4'-isomer in the diamine component of the reaction product is reduced.
There are a number of reasons why it is undesirable to use these high concentrations of strong acid in the condensation. Thus, the presence of the strong acid, particularly in the later stages of the condensation which are conducted at elevated temperature, represents a serious corrosion problem involving constant repair and maintenance requirements which contribute significantly to the overhead cost of operation of the manufacturing plants in which such processes are utilized. Secondly, the acid in question has to be neutralized, usually by means of aqueous sodium hydroxide, at the end of the reaction and disposal of the resulting neutral salt solution represents a severe problem because of the vast volumes of such material which are generated.
Processes have been described which eliminate the use of the strong acid catalysts and the necessity to neutralize the reaction products and substitute solid catalysts such as clay, zeolites and diatomaceous earth; see, for example, U.S. Pat. Nos. 3,362,979; 4,039,580; and 4,039,581. However, these processes give rise to products in which the 4,4'-isomer content of the diamine is substantially reduced in favor of the 2,4'-isomer and, in some cases, the 2,2'-isomer.
Attempts have been made to solve the problem of the necessity of neutralizing the strong acid catalysts at the end of the reaction by subjecting the final reaction mixture to solvent extraction to isolate the desired diamines and recycling the aqueous solution, containing the acid in the form of amine salt, to reaction mixture employed in a subsequent condensation. The strong acid is thereby recycled without the necessity to neutralize and convert to a salt requiring disposal. Typical of such processes are U.S. Pat. Nos. 3,996,283; 4,025,557; 4,087,459; 4,094,907; and 4,130,588. However, such processes only partially solve the overall problem since the strong acid is still present during the stage of the condensation which is carried out at elevated temperature.
We have now devised a process which offers a solution to both problems since it permits the total elimination or, at least, a significant reduction in the amount of strong acid present in the portion of the condensation carried out above room temperature. It follows that our process also results in elimination of, or significant reduction in, the acid which has to be neutralized at the end of the reaction. Our process, which will be described in detail below, involves extracting the product of condensation, at an intermediate stage prior to the final reaction at elevated temperature, to separate the intermediate condensation product from the aqueous acid reaction product containing the strong acid. The latter aqueous layer is reused in a subsequent aniline-formaldehyde condensation. The solvent extract is then subjected to the final stage of the reaction in the presence of a catalyst which can be a solid catalyst or a strong acid catalyst present in an amount which is significantly less than would be present if the acid used in the initial condensation had been retained for the second stage.
The process which is described herein is therefore differentiated from that described in British Specification No. 1,536,008. In the latter reference the solvent extraction of the intermediate stage in the condensation is carried out under conditions which result in the intermediate products being retained in the aqueous layer containing the strong acid, which layer is then carried forward to the next stage without any reduction or elimination of the strong acid.